Photovoltaic-type charger

ABSTRACT

The method of producing a photovoltaic panel ( 23 ) comprising elementary photovoltaic cells and intended for charging a low-voltage electrical energy accumulator ( 11 ) via a voltage converter ( 24 ) is characterized in that a minimum number of elementary cells is wired in series between the two terminals of the photovoltaic panel ( 23 ), this minimum number being defined in such a way that, under a given illumination, the knee voltage of the photovoltaic panel is greater than the threshold voltage of operation of the converter ( 24 ). The invention also relates to a device for charging a low-voltage accumulator via a converter comprising a photovoltaic panel obtained by this method.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method of producing a photovoltaicpanel comprising elementary photovoltaic cells and intended for charginga low-voltage electrical energy accumulator via a voltage converter anda device comprising a low-voltage electrical energy accumulator, astep-up voltage converter, a photovoltaic panel comprising elementaryphotovoltaic cells and intended for charging the accumulator via theconverter.

DESCRIPTION OF THE PRIOR ART

[0002] The field of the charging device according to the invention isthat of low-voltage accumulators for which there commonly existdirect-wired photovoltaic panels, that is to say ones wherein there issufficient internal serialization of the cells (or elementaryphotocells) to obtain, under solar exposure, a panel no-load voltagegreater than the voltage of the accumulator, and hence allowing directcharging of the accumulator through its electrical linkup to the panel.

[0003] More precisely, the invention relates to devices of thephotovoltaic type, intended to satisfy the charging of an electricaccumulator under highly variable circumstances of illumination. Inparticular, the devices using a method according to the invention lendthemselves to accumulator charging when the illumination availabledecreases greatly with respect to its nominal value.

[0004] Similarly, the invention applies not only to electrochemicalaccumulators, but also to the charging of electrical energy accumulatorelements in electrostatic form (with or without polarization), such ascapacitors or supercapacitors.

[0005] It is known not to directly use the output of photovoltaic panelsin energy collector devices. American patent U.S. Pat. No. 4,494,180cites very many scientific references to work undertaken in the searchfor an optimal operating point. This approach is known by theabbreviation MPPT: “maximum power point tracking”.

[0006] It is concerned with operating the panel according to voltage andcurrent drawn conditions that correspond to the power maximum. Such anoptimal point is represented by the reference (48) in FIG. 2 of thecited patent. It is situated at the knee of the current/voltagecharacteristic.

[0007] This patent describes in particular how to optimize the power insuch a way as to supply a motor directly, without even going via anaccumulator.

[0008] It will be pointed out that in this patent use is made of a DC/DCconverter of ratio N, as in the present invention, but it is a step-down(buck) rather than a step-up (boost) converter. It is concerned withvarying the apparent load seen by the panel, in such a way as to remainat the knee of the characteristic curve, current/voltage of the panel,whatever the illumination or the temperature.

[0009] Moreover, the same concern for apparent impedance matching of theload, seen by the panel, will be found in the earlier American patentU.S. Pat. No. 3,384,806.

[0010] American patent U.S. Pat. No. 3,696,286, is concerned withdriving at least one variable-frequency converter through the differencebetween the voltage delivered by at least one main panel, operatingunder load, and a reference cell, operating at no load while beingplaced under the same conditions of temperature and illumination as themain panel.

[0011] The converter structure is that of a conventional booster, butthe latter explicitly plays the role of power amplifier, rather thanvoltage amplifier, as represented in FIG. 1 and claimed. The aim is toautomatically decrease the power demanded by the load if the availablepower of the panel decreases: this again entails a matching system, ofMPPT type.

[0012] This patent provides for the use of several amplifiers, eachoperating at variable frequencies, to balance the output voltages ofpanels placed in parallel.

[0013] American patent U.S. Pat. No. 4,695,785 describes an MPPT deviceas regards variations in illumination and/or in temperature. The drivingof the device at the maximum power available is carried out byperiodically (and briefly) measuring the available short-circuitcurrent. This value is used to act on a switching regulator in order tomatch the power demanded by the load to the power available.

[0014] It will be pointed out that the abstract of the invention doesnot mention the boost or buck nature of the switching regulator. In thedescription of the preferred embodiment, it is however mentioned (col.4, line 34) that the components are those of a boost converter,corresponding to FIG. 1, but this characteristic is obviously notessential since it is clearly stipulated in the description of FIG. 1(col. 7, line 27) that this structure is taken as a current regulator.It is indicated in the next paragraph that other regulator setups couldequally well be used as converters of “blocking” or “forward” type.

[0015] In the known devices of the prior art, the aim is exclusively toget closer to the working conditions at the knee of the outputcharacteristic, current/voltage curve, of the panel, whereas the deviceof the invention is designed to preferentially utilize the zone in whichthe photovoltaic panel works as a current generator, that is to say thehorizontal part of the current-voltage curve, though the power is notoptimal, the aim being to yield as high a current as possible.

[0016] Let us add that the device of the invention is advantageouslyused, for example, for the supply of DC motors for the temporary andinfrequent moving of solar protections or the like, from theaccumulator. It applies to any other use in which it is desired to beable to profit equally from situations of low ambient luminosity andsituations of direct illumination of the panel, to allow the charging ofthe accumulator or the capacitor.

SUMMARY OF THE INVENTION

[0017] Thus, according to the invention, the method of dimensioning andusing a photovoltaic panel of given area intended for low-voltageelectrical energy accumulator charging is characterized in that theinternal structure of the panel is defined in such a way as to deliver,under given illumination, as high as possible a short-circuit currentprovided that the knee voltage of the panel or at least its no-loadvoltage, remains above the operating threshold of a step-up voltageconverter interposed between said panel and said accumulator.

[0018] The method of producing a photovoltaic panel according to theinvention is characterized in that a minimum number of elementary cellsis wired in series between the two terminals of the photovoltaic panel,this minimum number being defined in such a way that, under a givenillumination, the knee voltage of the photovoltaic panel is greater thanthe threshold voltage of operation of the converter.

[0019] The minimum number of elementary cells wired in series can bedefined in such a way that, under a given illumination, the no-loadvoltage of the photovoltaic panel is greater than the threshold voltageof operation of the converter.

[0020] The invention also relates to a device comprising a low-voltageelectrical energy accumulator, a step-up voltage converter, aphotovoltaic panel comprising elementary photovoltaic cells and intendedfor charging the accumulator via the converter, characterized in thatthe connection of the elementary cells of said panel is essentiallyparallel, the series connections being limited to what is necessary forobtaining a panel knee voltage or a no-load voltage of the panel whichis greater than the threshold voltage of operation of the converter.

[0021] According to a complementary characteristic, all the elementarycells of the panel are connected in parallel.

[0022] According to another characteristic, the step-up converter uses aconstant frequency and/or a constant duty ratio.

[0023] Let us note that the converter exhibits active periods ofoperation and inactive periods, which inactive periods are exploited inorder to charge with the aid of the panel, a storage capacitor arrangedupstream of the converter.

[0024] According to another characteristic, the converter comprises anoscillator furnished with an input disabling the operation of theconverter if the voltage on this input becomes less than a giventhreshold voltage, and then keeps it disabled as long as this voltagehas not regained a value greater than the threshold voltage.

[0025] According to two variant embodiments, the electrical energyaccumulator is of electrochemical type, or of capacitor orsupercapacitor type.

[0026] Other characteristics and advantages of the invention will emergefrom the description which follows in conjunction with the appendeddrawings which are given merely by way of nonlimiting examples.

DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a view illustrating the traditional device.

[0028]FIG. 2 is a diagrammatic view of the device according to theinvention.

[0029]FIG. 3 is a graphical representation of the charging curve of atraditional device and of the device according to the invention.

[0030]FIG. 4 is a diagrammatic view of the charging device according tothe invention and in particular of its step-up converter.

[0031]FIG. 5 illustrates the manner of operation and depicts theconductive times for the on/off switch of the converter.

DETAILED DESCRIPTION OF THE INVENTION

[0032]FIG. 1 represents a traditional charging device (12) for chargingan accumulator (11) of nominal voltage (UA), said device being of thephotovoltaic panel type, comprising a configuration of elementary cellsgiving a no-load voltage (UB) greater than the nominal voltage (UA) ofthe accumulator, for nominal illumination (WR).

[0033] For example, if the accumulator has a nominal voltage of 24volts, and if the no-load voltage of an elementary cell is 0.8 volts,the panel will have to contain at least 30 groupings of elementarycells, these groupings being linked in series. Inside each grouping, thecells are connected in parallel.

[0034]FIG. 2 represents a charging device according to the invention.

[0035] For one and the same panel area, and for the same accumulator asabove, the method according to the invention consists in rather choosinga configuration of the panel of cells (23) giving a no-load voltage ofthe panel (UB2) equal to the previous no-load voltage (UB) divided by amultiplier coefficient (N) for this same value of nominal illumination,and in interposing between the panel and the accumulator (11) a step-upvoltage converter element (24) with multiplicative factor equal to themultiplier coefficient (N), this latter (N) normally being constant,greater than unity and as large as possible.

[0036] In the preferred embodiment of a device according to theinvention, this converter (24) possesses at least one intermittent modeof operation, associated with an element for upstream storage ofelectrical energy.

[0037] A conventional device for charging an accumulator of nominalvoltage (UA) leads to choosing a panel such that the current/voltage(I/U) charging curve exhibits a no-load voltage (UB) greater than thenominal voltage (UA) of the accumulator, as represented diagrammaticallyin FIG. 3, by the curve (31).

[0038] To facilitate the description, it will be assumed for examplethat this voltage has been obtained by placing four groupings ofelementary photovoltaic cells in series in the panel.

[0039] At the optimum, as in all “MPPT” (maximum power point tracking)devices, the operating point is chosen, in the prior art, such that thenominal voltage (UA) of the accumulator corresponds to the knee (32) ofthe curve (31).

[0040] As a break with current practice, the invention advocates, forone and the same area of photovoltaic panel, that the value of themaximum strength (I max) of the short-circuit current be favored to thedetriment of the new no-load voltage of the panel (UB2). Relative tonormal dimensioning, the former no-load voltage (UB) will be divided bythe multiplier coefficient (N), (UB2=UB/N), whereas the value of thestrength of the current will be multiplied by the same factor. Curve(33) is then obtained.

[0041] In the example, the four groupings of cells of the panel are nowarranged in parallel: the output voltage of the panel is four timessmaller and is therefore less than the nominal voltage of theaccumulator, but the maximum current that the panel can provide hasbecome four times larger.

[0042] The invention does not seek specifically to track the maximumpower point (34), but will preferably use the range (35) within whichthe panel behaves as a current generator.

[0043] It will be pointed out that, for large values of the multipliercoefficient (N), the characteristic (33) tends moreover to becomerectangular, and that it is not possible to remain, in a stable manner,elsewhere than on the range (35). The knee voltage and no-load voltagethen have substantially the same value.

[0044] Of course, this panel dimensioning makes it necessary tointerpose a voltage booster device so as to allow the charging of theaccumulator. For simplicity, the same value (N) as above will be takenas multiplicative factor, but it would be equally possible to take adifferent value (M), provided that the no-load voltage multiplied by themultiplier coefficient (N) or by the coefficient of different value (M)is greater than the nominal voltage (UA) of the accumulator.

[0045] This photovoltaic panel dimensioning may seem very surprisingsince it leads to the need for a step-up converter that could veryeasily be dispensed with through normal dimensioning. However, itsbenefit resides in the capacity of the panel to provide current even incircumstances of weak illumination, this current being utilized, withina much larger dynamic swing (here around N times larger than bycomparison with the prior art).

[0046] In practice, the multiplier coefficient (N) can take as large avalue as possible (not necessarily integer). There is however atechnological limitation: it is imperative that the panel no-loadvoltage (UB) divided by the multiplier coefficient (N) remains greaterthan the minimum input voltage (UC) allowing operation of a step-upconverter based on semiconductors or the like.

[0047] Stated otherwise, the invention promotes the placing in parallelof the elementary cells of a panel of given area (S).

[0048] Ideally, the design according to the invention amounts to using aphotovoltaic panel comprising only cells in parallel for a given totalarea. The technology for producing the converter should then exhibit aminimum operating voltage (UC) which is less than the knee voltage or atleast than the no-load voltage of an elementary cell of the photovoltaicpanel.

[0049] Should these ideal characteristics not be achieved, thedimensioning of the panel according to the invention will simply have tolimit the number of series connections of groupings of cells to what isnecessary for obtaining a panel knee voltage, or at least no-loadvoltage (UB2), which is greater than the operating threshold (UC) of theconverter.

[0050] In the current state of converter technology, this operatingthreshold is of the order of 1 volt.

[0051] For a knee voltage of the elementary cells equal to 0.5 volts, apanel will for example be taken whose area is divided into threegroupings in series, interposing such a boost converter taken with amultiplier coefficient of 10 to apply the invention to the charging of a12-volt accumulator. According to the prior art, the panel area wouldhave been divided into at least 24 groups connected in series.

[0052] If it is assumed that an advance in photovoltaic technology givesa knee voltage equal to 1.5 volts for a photovoltaic elementary cell,then the application of the method according to the invention in respectof the charging of a 12-volt accumulator will consist in usingelementary cells in parallel over the entire area of the panel, whereasthe prior art would have distributed this area into nine or ten groupsconnected in series.

[0053] As a principle, if there is a total number of cells equal to T onthe area S of the panel, and if the threshold voltage of the converterrequires that P cells be placed in series in order to obtain a no-loadvoltage at least greater than the threshold voltage, and if we haveT=P.Q with Q an integer, then it is equivalent to producing: either Pgroupings of Q cells in parallel, these P groupings being connected inseries, or Q times the placing in parallel of P cells in series. Theinvention applies equally to either case. Technological reasons, inparticular regarding the balancing of the currents and/or voltages inthe various cells or groupings, in fact cause one or the other option tobe chosen. What is important is that P, according to the invention, willbe taken as small as possible so as to maximize Q.

[0054] The above principles are retained in the case where it isnecessary to charge a capacitor rather than an electrochemicalaccumulator. The voltage (UA) defined above as being the nominal voltageof the accumulator will now be defined either as the voltage at whichthe charging of the capacitor stops, or as the mean value of a chargingvoltage in an operating mode that tolerates a small excursion eitherside of this value.

[0055]FIG. 4 diagrammatically illustrates a device in which the panel(42) is dimensioned according to the method of the invention for thecharging of an accumulator (11). Other devices that are not representedmay be wired up in parallel with this accumulator.

[0056] The step-up converter (24) here comprises an inductor (43), anon/off switch (44) driven by an oscillator (45) whose duty ratio definesthe voltage boost, and operating for example at constant frequency, andfinally a diode (46) connected to the accumulator. Any other booststructure known to the person skilled in the art would be as suitable.

[0057] Such a setup allows for example the charging of a 6-voltaccumulator from a panel which will be chosen to deliver a no-loadvoltage of 2.8 volts. The duty ratio is then chosen to be 0.8, thisgiving a normal multiplicative factor of the converter equal to 5. It isnoted that all the conditions according to the invention are fulfilled,with a certain safety margin.

[0058] This setup also comprises an additional diode (47) and acapacitor (48), which makes it possible to produce an upstream storageelement. Furthermore, the oscillator (45) is furnished with an input(451) disabling the operation of the converter (24) if the voltage onthis input becomes less than a given threshold voltage (U1), and thenkeeps it disabled so long as this voltage has not regained a value atleast equal to (U2), greater than (U1). The value (U1) is taken equal toor slightly greater than the operating threshold voltage (UC) of theconverter.

[0059] In a first simplified version, the diode (47) could be replacedwith a conducting wire, and the capacitor (48) could be dispensed with.It is also possible to retain one without the other.

[0060] It should be noted that certain circuits, used under conditionsof low supply voltage, exhibit this dual-threshold behaviorspontaneously. Thus, the LT1613 circuit from Linear, that carries outall of the booster and oscillator function, naturally ceases operatingif its voltage becomes less than around 0.99 volts, and resumes for avoltage equal to around 1.1 volts.

[0061] The setup with diode and input capacitor is known to the personskilled in the art, this setup being such as to avoid invoking the paneldirectly when the on/off switch is conductive.

[0062] However, here it takes another function, in association with theinput (451), or the intrinsic manner of operation equivalent to thisinput, that of a manner of operation of relaxation type.

[0063]FIG. 5 illustrates such a manner of operation, depicting theconductive times of the on/off switch.

[0064] Under the conditions of low illumination, this manner ofoperation involves “puffs”, delivering a few pulses when the low currentdelivered by the panel has been able to charge the capacitorsufficiently to reach the threshold voltage (U2), thereby immediatelybringing about the operation of the converter, and hence the dischargingof the upstream storage capacitor, until (U1) is reached. The converterthen ceases operating until the upstream storage capacitor has been ableto recharge at a voltage (U2).

[0065] In certain cases of low charging current, arising from the panel,only a few pulses (51) are then available, and in the limit a singleswitching cycle of the on/off switch. These groups of pulses areseparated by a quiescent time (52) which decreases as the currentprovided by the panel increases. As soon as the current drawn becomessufficient, conduction becomes continuous, that is to say the on/offswitch operates permanently with the fixed frequency and the fixed dutyratio and the current charging the accumulator (or any device placed inparallel with the accumulator) is no longer interrupted.

[0066] With this configuration no electric charge provided by thephotovoltaic panel is lost.

[0067] In the preferred embodiment, the diode (47) is not used, so as toavoid an additional voltage drop.

[0068] Finally, use may also be made of a disabling input (452) in thecase where the output voltage would exceed the maximum voltage of theaccumulator at full charge.

[0069] In the case where it is a capacitor (or supercapacitor) (11) thatis charged with the device according to the invention, it will bepointed out that one benefits from transparent operation of the step-upconverter so long as, in an initial operating phase, the capacitor hasnot reached a voltage at least equal to the threshold voltage of theconverter. Specifically, almost the entire current delivered by thepanel passes through the inductor (43) and the diode (46) to charge thecapacitor (11), the capacitance of which is normally very large comparedwith that of the upstream capacitor (48).

[0070] The duty ratio of the converter can be chosen to vary in such away as to optimize the conductive time of the on/off switch (44) in thesubsequent phase of steady growth of the voltage across the terminals ofthe capacitor (11) during the charging thereof. The disabling input(452) is used to terminate the charging beyond a given value of voltage.

[0071] Of course, the invention is not limited to the embodimentsdescribed and represented by way of examples, but it also comprises alltechnical equivalents as well as their combinations.

1. A method of producing a photovoltaic panel (23) comprising elementaryphotovoltaic cells and intended for charging a low-voltage electricalenergy accumulator (11) via a voltage converter (24), characterized inthat a minimum number of elementary cells is wired in series between thetwo terminals of the photovoltaic panel (23), this minimum number beingdefined in such a way that, under a given illumination, the knee voltageof the photovoltaic panel is greater than the threshold voltage (UC) ofoperation of the converter (24).
 2. The method of producing aphotovoltaic panel (23) as claimed in claim 1, characterized in that theminimum number of elementary cells wired in series is defined in such away that, under a given illumination, the no-load voltage (UB2) of thephotovoltaic panel is greater than the threshold voltage (UC) ofoperation of the converter (24).
 3. A device comprising a low-voltageelectrical energy accumulator (11), a step-up voltage converter (24), aphotovoltaic panel (23) comprising elementary photovoltaic cells andintended for charging the accumulator via the converter, characterizedin that the connection of the elementary cells of said panel isessentially parallel, the series connections being limited to what isnecessary for obtaining a panel knee voltage or a no-load voltage (UB2)of the panel which is greater than the threshold voltage of operation(UC) of the converter (24).
 4. The device as claimed in claim 3,characterized in that all the elementary cells of the panel areconnected in parallel.
 5. The device as claimed in claims 3 or 4,characterized in that the step-up converter uses a constant frequencyand/or a constant duty ratio.
 6. The device as claimed in any one ofclaims 3 to 5, characterized in that the converter exhibits activeperiods of operation and inactive periods, which inactive periods areexploited in order to charge with the aid of the panel, a storagecapacitor (48) arranged upstream of the converter.
 7. The device asclaimed in claim 6, characterized in that the converter (24) comprisesan oscillator (45) furnished with an input (451) disabling the operationof the converter (24) if the voltage on this input becomes less than agiven threshold voltage (U1), and then keeps it disabled as long as thisvoltage has not regained a value at least equal to (U2), greater than(U1).
 8. The device as claimed in any one of claims 3 to 7,characterized in that the electrical energy accumulator is ofelectrochemical type.
 9. The device as claimed in any one of claims 3 to7, characterized in that the electrical energy accumulator is ofcapacitor or supercapacitor type.